Corrugated paper board is commonly prepared by first forming a corrugated element or medium by passing a cellulosic sheet between corrugating rolls forming a substantially sinusoidal or serpentine cross-section in the sheet. An adhesive is commonly applied to the tips of the thus formed sinusoidal portion called flutes and a noncorrugated or planar cellulosic liner is applied against the adhesive coated flutes of the corrugated elements as the corrugated sheet passes between a corrugating roll and a pressure roil. The resulting paper product having the corrugating medium on one side and the planar liner on another side is called a single-faced web. The single-faced element may be used as is in certain applications as a liner or buffer material within a container. More commonly adhesive is applied to the flute tips of the single-faced web and a second liner sheet is subsequently applied to the fluted medium in a “double facer” operation. The second liner sheet is exposed to conditions of heat and pressure during its contact with the adhesive. In practice, the sheet of corrugated cardboard most frequently encountered has two plane sides placed on each side of the corrugated medium. Depending on the specific strength desired, a sheet of corrugated cardboard may also be provided with a more complex structure, such as two corrugated medium and three plane surfaces, two outer ones and one inner one separating the two corrugated medium. This is referred to as double wall board.
Starch-based adhesives are most commonly used in the corrugating process due to their desirable adhesive properties, low cost and ease of preparation. The most fundamental of starch corrugating adhesives is an alkaline adhesive which is comprised of raw, ungelatinized starch suspended in an aqueous dispersion of cooked starch. The adhesive is produced by gelatinizing starch in water with sodium hydroxide (caustic soda) to yield a primary mix of gelatinized or cooked carrier, which is then slowly added to a secondary mix of raw (ungelatinized) starch, borax and water to produce the fully formulated adhesive. In conventional corrugating processes, the adhesive is applied (usually at between 25° and 55° C.) to the tips of the fluted paper medium or single-faced board, whereupon the application of heat and pressure causes the raw starch to gelatinize, resulting in an instantaneous increase in viscosity and formation of the adhesive bond. In other words, the adhesive is applied while relatively cool and then, subsequently, requires the application of high temperatures at the points of bonding to effect in situ gelatinization of the raw starch granules, a process that is lacking in both efficiency and economy. This method of forming corrugated board is commonly referred to as the “Stein-Hall process”.
While certain prior art “cold” corrugating methods using starch-based adhesives have been suggested in order to eliminate the need for heating systems required for setting the adhesive, such process have not proven to be commercially viable. See, U.S. Pat. Nos. 3,300,360, 4,374,217, 4,561,918, and Clyde H. Sprague, Institute of Paper Chemistry, “Development of a Cold Corrugating Process—Final Report” (May 1985). Use of heat/heating systems is still conventional in the corrugating arts.
There continues to be a need in the art for corrugating methods and means, including processes designed to eliminate or at least minimize the amount of heat/heating systems required to prepare good quality moisture/water resistant corrugated board. There is also a need in the art for novel corrugating adhesives that enable production of moisture resistant board and that can be run “cold,” i.e., without the required use of added heat.